1. Field of Invention
The present invention relates to a new method and an apparatus that is intended to be used in the medical and scientific fields concerned with vision, the eye, and perception. The invention provides a method for recovering information about phasic visual neurons, e.g., in humans. This information has importance in the diagnosis of pathologies of the visual system and in basic research concerned with understanding visual processing.
2. Description of the Background
Visual information received by the human eye is processed in the eye and then transmitted to the brain via about 1 million optic nerve fibers. In general, two broad classes of the optic nerves can be distinguished, those that respond phasically and those that respond tonically. The phasic fibers exhibit a burst of activity whenever there is a change in the color, luminance, etc., in the visual stimulus provided to the human eye. Otherwise, these fibers do not exhibit much activity. The tonic fibers are characterized by a continuous activity, that lasts as long as a stimulus is present. So, if the eye is looking at a red field which suddenly changes to green, many phasic fibers will exhibit a burst of activity in response to the change from red to green. After a change in color has occurred, however, the phasic fibers will remain silent once again. Some tonic fibers also respond to a change in color from red to green. The responses of tonic fibers, however, further differ from those of phasic fibers in that the tonic fibers maintain their activity for as long as the color green is present.
A recognized method for recording the activity of phasic optic nerves and the corresponding neurons they connect to in the visual system is by a microelectrode recording procedure. This method is the conventional technique used for recording the electrical activities of single neurons. Since this technique is invasive, however, it is not used routinely for the evaluation of the visual system in human subjects.
There are several non-invasive techniques currently being used that can provide some information about optic nerve function. One general technique is referred to as a "psychophysical" technique because it relies on an observer's visual perception and requires a voluntary, often verbal, response. The use of the psychophysical technique, however, is often limited, e.g., if the patient is not able to provide a voluntary response or if the patient's voluntary response is not reliable.
Another general technique is the one referred to as "Visual Evoked Cortical Potentials" (VECP). This technique relies on the recording of visually evoked "brain waves" that are measured by placing contact electrodes on the scalp of a subject. The electrical waveform of the responses measured is often complex and, therefore, difficult to interpret.
A third technique is referred to as the pattern-electroretinogram technique. This technique measures electrical signals within the eye believed to originate from nerve cells whose axons form the optic nerves of the eye. This technique, however, requires the placement of an electrode on the surface of the eye of the subject. The electrode does produce mild discomfort which can affect the patient's compliance with the test. In addition, the responses obtained are generally very small and therefore, hard to interpret.
U.S. Pat. No. 3,777,738 to Sugita et al. provides a method and apparatus for diagnosis using the pupillary light reflex to measure the response of the retina or optic nerves. This patent does not isolate the activity of phasic visual neurons, nor does it provide a method to evaluate functions of the phasic visual neurons such as color discrimination, spatial resolution, etc. The prior art patent does, however, provide a means for testing different areas of the patient's visual field, but the visual field assessment differs from that envisioned in the present patent. The prior art patent is concerned with the general loss of visual field capacity not with specific losses of phasic-type neuron function or with losses in color discrimination, spatial resolution, or other specific visual functions.
U.S. Pat. No. 4,953,968 to Sherwin et al. involves the assessment of evoked visual responses and provides a method to evaluate a number of visual functions including color sensitivity, contrast sensitivity, etc. The responses referred to in the prior art patent are, in contradistinction to the present technology, associated with visually evoked potentials that are EEG responses from the visual cortex of a subject. The prior art patent neither pertains to the evaluation of pupillary responses, nor to the use of pupillary responses to obtain information about visual function. Furthermore, the patent does not provide a method for isolating the activity of phasic visual neurons. The method of the prior art patent thus differs from the invention disclosed and claimed herein.
U.S. Pat. Nos. 3,533,683 to Stark and 4,850,691 to Gardner et al. relate to the measurement and recording of the pupillary diameter and latency of changes under various conditions. These prior patents are not concerned with the evaluation of visual function using the pupillary response.
Thus, the use of the pupillary light reflex to recover information about optic nerve function in humans has been known. Up to the present time, however, there does not exist a method for recovering information about a specific class of optic nerves, i.e., the phasic type, from the pupillary light reflex. The method using the pupillary light reflex described below affords numerous advantages over alternative approaches.